Generally, reticulated structures are made of a network of two fundamental structural elements, nodes and interconnecting linear members. When the linear members consist of cylindrical tubes, connections to the nodes are generally pinned due to the configuration of connection mechanisms. Systems utilizing interlinked networks of pinned connections are commonly employed in the design, engineering, fabrication, and erection of numerous reticulated structures, including, but not limited to, domes, building facades, towers, stadium covers, bridges, and various applications of trusses.
In general, a typical pinned connection system employs a generally spherical node connected to a plurality of tubular framing members. The node has openings to receive parts, such as bolts, of connection assemblies, and the ends of tubular framing members are welded to other parts, such as an end cone, of the connection assemblies. The ends of the tubular framing members may be tapered to simplify or enhance the connection. By connecting each spherical node to a plurality of tubular framing members with connection assemblies, extensive networks of nodes and framing members may be interlinked to form a variety of reticulated structures. Many variations of this common "tube and ball node system" employ nodes attached to flat-faced connection assemblies to which tubular framing members are directly welded.
One specific type of tube and ball node system uses a spherical node with a plurality of rounded openings through which a bolt or pin is inserted and fixed therein. A funnel-shaped sleeve having a hollow cylindrical base is positioned between the spherical node and a hollow cylindrical framing member. The end of the framing member is properly swaged to conform to the funnel shape of the sleeve top. The swaged end of the framing member and the sleeve top are welded together, and a bolt is inserted through the framing member, extending through the sleeve base, and fixably attached, by threading to the node. An externally accessible collar, which is rotationally fixed to the bolt is provided to tighten the bolt. In this manner, the framing members are each individually attached to the spherical nodes and the reticulated structure is formed. In these configurations, shear, tensile, and compressive stresses are borne by the bolts. In some cases, the collar bears the compressive stresses. Generally, the bolts react well in tension but shear and compression can be problematic.
An interlinked network of pinned connection systems can be employed in a variety of ways in reticulated structures. These uses range from primarily acting as a load bearing structure to being an aesthetic addition to a building facade. The structural purpose of the network guides the determination of what compression and tensile forces will act upon the system. The network is designed to withstand the calculated magnitude and direction of compression and tensile forces. The effective design of such networks requires the development of an appropriate model of the system of pinned connections being employed in the structural design. Preferably, the model should be as simple as possible without compromising engineering accuracy. Furthermore, when employing cylindrical framing members, the connection system being used should strive to optimize the compression bearing characteristics common to cylindrical tubes.
Typical tube and ball node systems have some deficiencies that become apparent in the design, fabrication, and field erection of reticulated structures. Each member-node connection point within a standard tube and ball node system usually is modeled as a three part system comprised of a hinge at the node, a short flexible member representing the bolt and lastly the tube. Because the framing member often tapers into and is welded to a funnel-shaped sleeve which then connects into the node, a three part model is required to measure, with sufficient accuracy, the existence of a moment at the member-node connection point. The three part model obtains the accuracy necessary through the incorporation of a short flexible member and hinge. This three part model is problematic because of the complexity it introduces into reticulated structure designs.
To utilize simpler models would require the structure to have a substantial moment resisting capability at the point of connection between framing members and nodes, so that the short flexible member representing the bolt and/or collar with hinge could be deleted. Tube and ball node systems, however, fail to sufficiently resist moments at this connection because current designs require framing members to taper into a sleeve, collar and bolt. The reduction in the cross-sectional area of framing members at the connection introduces into the design an additional structural element, modeled as a hinge, and typically prevents moment transfer at the point of connection. Thus, under load, the pinned connection rotates slightly. As might the short flexible member without moment transfer. This also inhibits the application of loads to the framing members, and therefore, loud speakers, light assemblies, and other equipment must be attached to the nodes. Furthermore, when dealing with a cylindrical member, a reduction in cross-section at the connection diminishes its compression bearing characteristics.
To be practically employed in structural design, elements of a connection system should be relatively easy to manufacture. In tube and ball node systems, spherical nodes are difficult to manufacture and are often limited in size. Tube and ball node systems also have limitations in the type of material which can be used. Although polymers, plastics, aluminum, and a variety of composites may have the requisite strength, cost, and aesthetic design characteristics to be desirable for use in a reticulated structure, such materials are often sensitive to welding. The use of welding in the fabrication of a reticulated structure is costly and time consuming. It requires quality control measures, uses raw materials, and requires skilled workers. The use of welding in tube and ball node systems decreases the allowable stresses certain materials can bear and eliminates them from consideration for particular applications.
Finally, because of the intricate nature of many reticulated structures, field erection and assembly can be both difficult and costly. The erection of these structures is a rigid unforgiving process, and, often, the erection of a reticulated structure progresses to the point where two nodes, which are not yet connected by a framing member, are sufficiently fixed in place by a prior series of assembled connections such that connecting a framing member between the two fixed nodes requires forcibly moving the nodes apart to connect the tube. This is time consuming and can damage the structure.
Thus, it is desirable to have a connection system which can be simply modeled as an elongated member fixably attached to a nodal point, without having to incorporate a hinge and/or short flexible member to account for the transfer of forces at the point of connection. It is also desirable to develop a connection system that enables a cylindrical member to be used without significantly reducing its cross sectional area at the point of connection. Furthermore, it is desirable to have a connection system which does not require welding to attach framing members to nodes and provides a built-in clearance between the nodes to facilitate the insertion of framing members between two fixed points. It is also desirable to have a connection system design where the elements of the system are easier to fabricate, have a more flexible range in available material types, and are not limited in shape. It is further desirable to avoid loading bolts with shear and compressive stress.